1. Field of the Invention
This invention is directed to the formation of CF.sub.4 gas in general and to the formation of CF.sub.4 gas in a controlled manner from Teflon in particular.
2. Description of Prior Art
The production of large ultra-pure metal fluoride single crystals suitable for use in high power laser applications, was described in U.S. Pat. No. 3,649,552 issued to Applicant Morton Robinson herein and Donald M. Cripe. In the 3,649,552 Patent, HF gas in a He carrier is shown to be an effective scavenger of oxide impurities found during the growth of fluoride single crystals in a modified Stockbarger type furnace. The furnace was operated at temperatures on the order of 1500.degree. C. and HF gas was obtained from a cylinder. This process, which later came to be recognized as a relative atmosphere processing (RAP) process had as its principal objective the suppression of hydrolysis during crystal growth in accordance with the following expression: EQU F.sup.- (c) + H.sub.2 O(g).revreaction. OH.sup.- (c) + HF(g) (1)
The forward direction of Eq. (1) shows hydrolysis of the condensed phase (c) by H.sub.2 O molecules in the gas phase (g). The reverse direction of Eq. (1) shows the removal of the hydrolysis product, OH.sup.-, an undesired anion impurity found in, and on the surface of, metal fluoride crystals.
From the mass-action principle, it follows that in order to effect a very low ratio of OH.sup.- (c):F.sup.- (c), one must provide a low ratio for the pressures of the sources in the gas phase, P(H.sub.2 O)/P(HF). This latter ratio, the sole material parameter of the growth process, is called the RAP-index.
The use of HF/He, as taught in the 3,649,552 Patent, leads to a low RAP-index by increasing the denominator of the source pressures discussed above. Thus, the method is limited by the steady state value of the numerator, P(H.sub.2 O).
A similar use of HF as a getter or scavenger during a crystal growth process is disclosed in U.S. Pat. No. 3,769,230 issued to Applicant Morton Robinson and Donald P. Devor in October of 1973. Like the 3,649,552 Patent, this patent teaches the use of a HF/He gas purge at some elevated temperature to eliminate anion impurities. It differs significantly from the teachings of the instant invention in that it does not address the production of CF.sub.4 gas as a reliable cheap fluoridizing agent, and it does not suggest a way to achieve a lower RAP-index to increase the additional scavenging effect needed to produce ultra-pure metal fluoride crystals.
An HF atmosphere was also disclosed, in U.S. Pat. No. 3,959,442 issued to the Applicants herein on May 25, 1976, to be effective for promoting the congruent melt crystal growth of laser emission compounds.
In order to provide additional scavenging, additional sources of reactive fluorine compounds were sought. Mixtures of HF and CF.sub.4 in He proved to be effective but commercially available CF.sub.4 is very expensive. Therefore, alternative methods of producing CF.sub.4 are needed.
Crystal growers in the USSR have used Teflon (the well-known trademark for polytetrafluoroethylene resins) in their crystal growth apparatus to generate C.sub.2 F.sub.4. However, they claim that C.sub.2 F.sub.4 did not decompose into CF.sub.4 and C until 1200.degree. C., an undesirable high process temperature for the growth of alkaline-earth flourides which occurs at temperatures above 1200.degree. C. In such cases, workers have observed that carbon becomes intimately mixed with the melt. Consequently, the melt is useless for crystal growth (see E. N. Chernevskaya and Z. N. Korneva, "The Production of Fluorite Crystals in an Atmosphere Containing Fluorine," Sov. J. Opt. Tech. 39, 213(1972)).
Applicants herein know of no additional art related to the decomposition of Teflon to generate CF.sub.4 or dissociable fluorine compounds or of art related to the use of fluoridizing agents to achieve a low RAP-index during the growth of metal fluoride crystals.